Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Treatment with particular chemical
Reexamination Certificate
1997-04-11
2003-04-29
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Treatment with particular chemical
C162S076000, C162S078000
Reexamination Certificate
active
06554958
ABSTRACT:
BACKGROUND OF THE INVENTION
1.1 Technical Field
The present invention relates to a process for the delignification and bleaching of chemical paper pulps.
1.2 Description of The Related Art
Chemical paper pulps or chemical pulps are those obtained by cooking lignocellulose materials, in particular wood, in the presence of chemical agents, such as sodium hydroxide, for Kraft, sulfite or bisulfite pulps.
All types of wood may be suitable. Mention may be made, by way of example, of softwoods, such as the various species of pines and firs, or hardwoods, such as birch, poplar, beech and eucalyptus.
Chemical pulps obtained by cooking are generally subjected to a number of delignifying and/or bleaching treatment stages. The first stages, which consist of completing the delignification resulting from the cooking, are followed by the bleaching stages.
On completion of these delignifying and bleaching treatments, the pulps should generally exhibit a high whiteness level and a very low Kappa number while retaining good mechanical properties, that is to say, the pulps are without significant degradation of the cellulose. This degradation can be detected by measuring the degree of polymerization (DP) of the pulp. The DP should remain as high as possible.
Thus, patent application WO95/31598 describes a delignification and bleaching process comprising a stage of treatment with hydrogen peroxide in the presence of alkali metal silicate at a temperature (T) greater than 100° C. and at a pressure greater than 1.5 times the saturated vapor pressure of water at temperature T. As indicated in Table I of this application, the presence of silicate is necessary in order to obtain a delignified pulp having both a high whiteness level and a high DP.
Moreover, the article by Messrs. Bertel Stromberg and Richard Szopinski entitled “Pressurized Hydrogen Peroxide Bleaching for Improved TCF Bleaching” presented at the 1994 International Pulp Bleaching Conference shows that bleaching by pressurized hydrogen peroxide results in substantial degradation of the cellulose.
Contrary to these preconceived ideas, it has been discovered that chemical pulps can be treated under pressure with hydrogen peroxide and in the absence of silicate according to the present invention.
DESCRIPTION OF THE INVENTION
In fact, a new process has been discovered for the simultaneous delignification and bleaching of a chemical pulp by hydrogen peroxide to obtain a highly whitened pulp which has retained a good degree of polymerization.
This process is characterized in that, after pretreatment with a complexing or sequestering agent for transition metals, in particular, manganese, the pulp is subjected to treatment with hydrogen peroxide in one or more stages at a temperature T greater than 100° C., at a pressure greater than 1.5 times the saturated vapor pressure of water at the temperature T, in the presence of a polymer comprising units of formula (I) and/or (II):
in which R
1
and R
2
, which are identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 3 carbon atoms and M represents a hydrogen atom, an ammonium group, an alkali metal, an alkaline-earth metal, or mixtures thereof and in the presence of a compound A chosen from potassium hydroxide, sodium hydroxide and alkali metal carbonates, alkaline-earth metal carbonates, or mixtures thereof.
Use is advantageously made of unsubstituted poly(&agr;-hydroxyacrylic acid), i.e., R
1
=R
2
=H, the corresponding polylactone and/or the salts of unsubstituted poly(&agr;-hydroxyacrylic acid). The potassium, sodium, magnesium and calcium salts are advantageously chosen from these salts, i.e., M=K, Na, Mg, Ca, or mixtures thereof.
The average molecular weight of the polymer comprising units of formula (I) and/or of formula (II) is generally between 1,000 g/mol and 800,000 g/mol and preferably between 2,000 g/mol and 100,000 g/mol.
The polymer comprising units of formula (I) and/or of formula (II) is known as a stabilizing agent for peroxide solutions (See: GB 1524013, FR 2601025). It can be prepared by using the methods described in French Patents FR 2237914, FR 2237916 and FR 2628745.
The compound A is advantageously chosen from the carbonates, such as sodium carbonate, potassium carbonate, magnesium carbonate and calcium carbonate, because the process according to the present invention offers, in this alternative form, the advantage of not producing any liquid effluent, i.e., it is Totally Effluent Free (TEF). Thus, after evaporation of the wash water (the washing is described hereinbelow) and incineration of the organic matter, the alkali metal or alkaline-earth metal carbonate is easily regenerated, without requiring a stage of causticizing with lime. Sodium carbonate is preferably used.
Unless otherwise specified, the amounts of the products and reagents according to the present invention are always expressed as percent by weight with respect to the weight of the dry matter of the pulp.
The consistency of the pulp is expressed as percent by weight of dry matter with respect to the total weight of the pulp.
In general, the amount of polymer used is between approximately 0.05% and approximately 1.5% by weight, preferably between approximately 0.1% and approximately 1%, and more preferably between approximately 0.2% and approximately 0.5%.
Depending on the starting pulp used and the amount of hydrogen peroxide involved, the compound A is added in the proportion of 1% to 15% by weight and, preferably, of 6% to 10% for the carbonates.
The amount of hydrogen peroxide used can vary from 0.5% to approximately 10% by weight. Use is preferably made of an amount of hydrogen peroxide of between approximately 1% and approximately 4% and, more preferably, between approximately 1.5% and approximately 2.5%.
During the stage of treatment with hydrogen peroxide, use may additionally be made of a sequestering agent, such as DTPA (sodium diethylenetriaminepentaacetate) or EDTA (sodium ethylenediaminetetraacetate), preferably in an amount of less than 0.2% by weight.
According to the present invention, the pulp, before treatment with hydrogen peroxide, can be subjected to one or more stage(s) of delignification by ozone and/or chlorine dioxide and/or organic peracids and/or inorganic peracids and/or oxygen, as known in the paper industry. Oxygen is preferably used.
On completion of the delignifying treatment, the pulp can be washed once or several times with hot or cold water.
Any pulp having a Kappa number (per SCAN standard Cl-59) not exceeding 17 before treatment with hydrogen peroxide is particularly suitable. MCC (modified continuous cooking) pulps, EMCC (extended modified continuous cooking) pulps and Super Batch pulps, the Kappa number of which, after cooking, can reach values as low as 15-18 for softwoods and 13-15 for hardwoods, are advantageously used.
The complexing or sequestering agent for transition metals used in the pretreatment can be chosen from DTPA, EDTA, phosphoric acids or salts of phosphoric acids. It is also possible to combine a number of agents in order to increase the efficiency of the pretreatment with respect to a greater number of metals.
The amount of complexing or sequestering agent is generally between approximately 0.05% and approximately 1% by weight. Use is preferably made of an amount of between approximately 0.1% and approximately 0.5%.
The temperature of the pretreatment is generally from 20° C. to 100° C. and preferably between approximately 60° C. and approximately 90° C.
The duration of the pretreatment with the complexing agent is generally from 1 to 30 minutes and preferably from 5 to 15 minutes.
The consistency of the pulp during the pretreatment can vary within limits ranging from 1% to 25% by weight. A consistency of between 5% and 15% is preferred.
Although the pretreatment with the complexing agent can be carried out in a medium at acidic pH, it is preferable to carry out the pretreatment at basic pH. The pH is advantageously greater than 7 and less than or equal to 12.5. A pH of between 8 and 10 is pa
Alvo Steve
Atofina
LandOfFree
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